In an axial disc brake as known from WO91/14032, intended for the weft yarn rotatingly withdrawn from a storage drum of a yarn feeding device by a textile machine, in one embodiment the mutually contacting braking surfaces are provided exactly perpendicular to the axis of the storage drum. In a further embodiment, the braking surface provided on the front side of the storage drum is oriented precisely perpendicular to the storage drum axis, while the other braking surface is formed by a convexly rounded outer edge of a funnel-shaped braking element having a funnel angle of almost 180.degree.. The braking surface which is perpendicular to the storage drum axis in a further embodiment is unitarily formed with the storage drum. In case of an overhead-withdrawal of the yarn from the storage drum the yarn is rotating like the pointer of a clock and then is entering essentially axially and from the outer side in between the braking surfaces. The yarn is braked by the braking surfaces and then is withdrawn approximately centrally via an opening provided in one of the braking surfaces. In the embodiment having braking surfaces oriented exactly perpendicular to the storage drum axis the yarn is deflected twice by 90.degree.. Such strong deflection may mean under undesirable conditions high load for the yarn and might lead to undefined friction conditions. Centering one of the braking elements on the axis of the storage drum is without problems because of the orientation of the braking surface exactly perpendicular to the storage drum axis. However, a lateral displacement of the braking element, e.g. caused by an axial component of the yarn tension exerted onto said brake element, may cause a disturbance of the mutual contact between both braking surfaces. With the strong deflection of the yarn when entering in between the braking surfaces the friction force depends on the deflection angle and increases by an exponential function depending on said deflection angle and might be superimposed onto the actual braking effect between the braking surfaces. This might lead to a high yarn breakage quota in case of vulnerable yarn qualities like cheap woolen- or cotton-yarns of low quality.
In a yarn brake of a yarn feeding device as known from EP-A-652312, a braking surface having a circumferentially continuous extension and the shape of a frusto-cone coat directly contacts the rounded withdrawal rim of the storage drum of said yarn feeding device. The axis of said frusto-cone coat of said braking surface approximately coincides with the axis of said storage drum. The other barking surface is defined by said rounded withdrawal rim of said storage drum and is designed--in an axial section of said storage drum with a curvature following a circle arc section. The center of said circle arc section is situated inside said storage drum, but is positioned a significant radial distance from the storage drum axis. The frusto-conical braking surface is formed by a deformable band. In case of precise centering of the band in relation to the storage drum axis, the contact zone between the braking surfaces is a full circle. In case that the yarn is forced through the contact zone and/or a small lateral displacement or tilt of the axis of the frusto-cone coat occurs in relation to the storage drum axis, the form of the contact zone changes into a geometrical form other than a full circle.
It is an object of the invention to create an axial disc brake of the kind as disclosed above as well as a yarn feeding device with which a yarn geometry can be achieved which is advantageous for the yarn, avoids strong deflections, and which is accompanied by a correct mutual centering of the braking surfaces with proper mutual contact relationship in their contact area between the braking surfaces.
Said task can be achieved according to the invention by providing an axial disc brake having a pair of form stable braking surfaces axially yieldably pressed against one another with predetermined contact pressure. The braking surfaces define an inlet gap in the area of an outer circumference of the disc brake, with the downstream braking surface having a central withdrawal opening. A braking element is provided which is movably supported at least in the axial direction. One of the braking surfaces is spherically convex with the sphere center thereof located approximately along the axis of the disc brake, and the other braking surface is conical with a cone axis approximately coinciding with the axis of the disc brake. The generatrix of the cone of the other braking surface in the contact zone of the respective braking surfaces is a tangent to the generating circle of the spherical other braking surface. The above disc brake can advantageously be provided on a yarn feeding device for a textile machine, particularly a projectile or gripper weaving machine, the yarn feeding device having a yarn storage drum for overhead yarn withdrawal, which storage drum is provided frontally with a withdrawal region defining a circular yarn guiding surface, wherein the disc brake is arranged coaxially with the storage drum adjacent a front end thereof.
Due to the cooperation between the spherical braking surface and the conical braking surface in contact therewith, the mutual contact zone is inclined, in which contact zone the withdrawn yarn is braked. The withdrawn yarn is drawn from the exterior towards the central withdrawal opening and is simultaneously rotating like the pointer of a clock about the axis of the disc brake. As a result, a very advantageous yarn geometry with weak deflection is achieved. The incoming yarn enters into the contact zone in a straight manner and is then deflected only gently behind the contact zone and into the withdrawal opening. In addition, constant contact relationships and a proper mutual centering of the braking surfaces result from the cooperation of the braking surfaces, since the contact zone remains a full circle even with deviations of the cone axis in relation to the axis of the disc brake or the storage drum, and even when the spherical braking surface becomes displaced. The diameter of said full circle remains unchangeable due to the geometrical conditions. Due to the advantageous yarn geometry the danger of yarn breakages is small since the yarn coming from the yarn guiding surface without deflection enters the disc brake while rotating like the pointer of a clock and is only gently deflected behind the contact zone of the braking surfaces. The reduction of the respective deflection angle and the avoidance of any deflection in the entrance area of the disc brake has a particularly positive effect on the yarn, because of the strong exponential influence of the deflection angle. The yarn is braked practically only in the contact zone such that the constant braking effect of the braking surfaces leads to a very advantageous, almost constant yarn tension in the outgoing yarn. Occasionally occurring relative displacements between the breakage surfaces do not change the contact- and braking relationships between the braking surfaces, since the full circle of the contact zone is maintained constantly as the braking surfaces are apt to automatically mutually align each other in a correct way. Also manufacturing- or assembling-tolerances are without significant influence on the braking effect due to the perfect cooperation between the conical braking surface and the spherical convex braking surface. The spherical convex braking surface is provided at the entrance side of the yarn. The center of the sphere is positioned in or along the axis of the disc brake or the storage drum, respectively.
In one embodiment, the tangent corresponding to the conical braking surface defines an angle of less than 90.degree. with respect to the brake axis, and the orientation of the tangent essentially determines the entrance-direction of the yarn such that the yarn follows an optimal yarn geometry without deflection at the entrance side and with gentle deflection at the withdrawal side only.
In another embodiment, a concentric circular yarn guiding surface is provided upstream of the disc brake and has an outer diameter which is larger than the outer diameter of the brake. The contact zone between the braking surfaces is axially spaced from the yarn guiding surface such that a theoretical extension of the tangent in the contact zone is approximately tangent to the yarn guiding surface. The disc brake brakes on a smaller diameter than the diameter of the yarn guiding surface. The yarn thus follows an optimal path without deflection into the disc brake.
In yet another embodiment, the front side of the storage drum itself defines the spherical braking surface or supports a braking body having the spherical braking surface. The other brake element is supported in a holder and is axially movable. A brake-drive actuates the braking element with an actuation force which is suitable for the desired braking effect, and in a yieldable way, such that the braking element is apt to fulfill all movements necessary during the dynamic phase of the yarn withdrawal for automatically maintaining a constant braking effect. Space is saved in the axial direction of the storage drum since the brake elements are optimally provided close to the storage drum.
In still another embodiment, the braking body is supported at the front end of the storage drum such that same is capable of tilting to all sides about a support center, and is additionally supported with radial clearance. The braking body is apt to automatically adapt to the respective relative position of the braking element, e.g. by tilting motions or by radial displacement motions.
The braking body can be made from metal or plastic and has the shape of a sphere with an outer edge flange and a central depression. The braking body is thus simple to manufacture and guarantees long periods of time wherein the brake effect is constant. The edge flange and the centrally provided depression increase the form staiblity of the brake body and allow its simple fixation. The edge flange extending rearwardly defines a safety factor by means of which entrance of the yarn behind the brake body is prevented in a structually simple way.
The brake element can be made from metal or plastic and has the shape of a funnel with an outer edge flange and a central yarn funnel having a funnel angle which is smaller than the cone angle of the conical braking surface. The brake element is simple to manufacture and guarantees long periods of time wherein the braking effect is constant. The outer edge flange increases form stability of the braking element and prevents an accidental entering of the yarn behind the braking element. The yarn funnel leads to a gentle withdrawal motion of the yarn and can advantageously serve to there receive a yarn eyelet for optimum friction- and deflection-relationships.
A particularly advantageous yarn geometry can be achieved with a cone angle range of the conical braking surface between 90.degree. and 160.degree.. An optimum value of the cone angle is about 1200 in order to achieve symmetrical yarn geometry.
To enable an instantaneous and sensitive response of the disc brake, a diameter range of the circular contact zone of the braking surfaces is between 10% and 50% of the maximum diameter of the yarn guiding surface. Having such a relatively small diameter in the contact zone makes it possible to manufacture the braking elements of lightweight material. Advantageously, the yarn rotating along the yarn guiding surface like the pointer of a clock runs freely between the yarn guiding surface and the contact zone over a relatively long distance such that inherent movements of the yarn within the distance are not transferred significantly into the contact zone where they could be detrimental for the braking effect. Additionally, the long free yarn distance between the yarn guiding surface and the contact zone results in a strong pointer rotational movement so that the yarn is drawn in correctly and radially into the contact zone independent from its linear yarn speed.
In another embodiment, the brake drive is provided with at least one spring element which contacts the braking element in a circle which is substantially axially aligned with the contact zone of the braking surfaces. The contact force between the braking surfaces is precisely selectable by means of the spring element. The braking element may, if necessary, yield (also when a knot is passing). It is of particular importance that the spring element acts approximately in the same diameter as the diameter of the contact zone such that a straight force transmission results from the spring element into the contact zone.
In still another embodiment, a membrane is fixed to the edge flange of the braking element and prevents the entrance of the yarn behind the braking element. Furthermore, the braking element can be lifted from the braking body by means of the membrane and a suction actuation of a suction chamber in order to allow a comfortable threading of the yarn (manually or by means of a pneumatic threading device). The membrane does not have any detrimental influence on the normal braking effect. However, the membrane may contribute to the centering of the braking element.
In a further embodiment, the brake drive is provided with an electrically, electromagnetically or pneumatically actuatable return drive which counters the actuation force of the spring element and is connected to the braking element. Such disc brakes are suitable for projectile or gripper-weaving machines in order to vary the braking effect during each insertion cycle depending on the weaving cycle. The return drive disengages or relieves the axial disc brake in operational phases during which no or only a minimum braking is necessary, e.g. at the start of an insertion, after the changeover phase or at the end of the insertion. By means of the spring elements a basic braking effect is adjusted which then can be lowered or modulated by the return drive.
The braking element, and also the braking body, may be constructed of lightweight metal and carry, preferably at the braking surfaces, a wear-resistant coating.
Since the pointer movement of the yarn entering the disc brake is a basic prerequisite for an optimum function of the disc brake, it is very advantageous to combine the axial disc brake with a second upstream braking device, e.g. a braking device acting in the region of the yarn guiding surface of the storage drum in order to stabilize the yarn with low tension in the entrance area into the axial disc brake. The second braking device may be a braking ring equipped with bristles or the like cooperating with the storage drum by contact. The geometrical forms of the spherical or conical braking surfaces need not be exactly derived from a sphere or a cone. Since the relative displacements of the braking surfaces have only relatively small magnitudes, an optimally operating axial disc brake also can be achieved in case both braking surfaces slightly deviate from a precise geometrical sphere- or cone-shape.